Solder flux



Patented July 28, 1942 SOLDER FLUX Mike A. Miller, New Kensington, Pa.,assignor to Aluminum Company of America, Pittsburgh, Pa., a corporationof Pennsylvania No Drawing. Original application December 29,

1939, Serial No.

2,266,060, dated December 16, 1941.

311,599, now Patent No.

Divided and this application March 13, 1941, Serial No.

8 Claims.

This invention relates to a solder flux which is useful for the softsoldering of metals. This application is a division of my copendingapplication Serial No. 311,599, filed December 29, 1939, which issuedDec. 16, 1941, as U. S. Pat. No. 2,266,060. The flux compositionsdisclosed but not claimed herein are claimed in my above-mentionedcopending application and copending applications Serial Nos. 383,171 and383,173, filed March 13, 1941.

The soldering art is classified into hard or soft soldering, dependingon whether a high or low melting point solder is employed. Generally,soft soldering is done with solders which melt below 700 F.,-whereas thesolders used for hard soldering melt at about 1300 F. or higher,according to the 1939 edition of the American Society for MetalsHandbook, page 1205. In order to obtain a good contact between thesolder and metal being joined, it is necessary to use a flux forpreparing the metal surface that will attack said surface at atemperature below the melting point of the solder. In the case of lowmelting point solders, a problem arises in securing satisfactory lowmelting point fluxes. While some salt fluxes meet the requirement ofhaving a low melting point, they are undesirable from the standpoint ofleaving a residue which, if not removed, promotes corrosion of thesoldered joint. To overcome this disadvantage, it has been proposed thatnoncorrosive organic compounds be employed, but these are often toosluggish in their action on the metal to be commercially useful. Theyalso have little or no effect upon the spreading of the molten solder,which is a highly desirable characteristic in a flux. This condition hasbeen met in part by combining a hydrohalide acid with an organiccompound. When such an addition compound is heated, the acid componentis released and attacks the metal. This action of the acid may be toosevere and cause an unnecessarypitting or localized attack on the metalsurface,,with a resultant non-uniform bonding of the solder to themetal. Furthermore, some acid components are exceedingly irritat ng tothe operator, if not actually poisonous, and hence must be used withconsiderable care. I

It is an object of my invention to provide an organic flux suitable foruse in the soft soldering of metals that is non-corrosive when left incontact with the metal being soldered, and yet which mildly butuniformly attacks the metal surface during the soldering operation.Another object is to provide a flux that is easy to handle. Stillanother object is to provide a low melting point flux which promotes thespreading of the molten solder. These and other objects will becomeapparent from the following description of my invention.

My invention is based upon the discovery that when boron trifluoride iscombined with a certain group of organic compounds a very satisfactorysoft soldering flux is obtained. More particularly, I have found thatboron trifluoride in combination with unmodified alcohols,alcohol-ethers,

cyclic ethers, ketones, organic esters, organic trifiuoride is notaccurately known in all cases,

it nevertheless appears that some kind. of compound is frequently formedwhich may be referred to as an addition compound. Whether or not acompound exists, I have found that the addition of boron trifluoride tothe above named classes-of organic compounds greatly enhances thefluxing characteristics of the compound, and in some cases it may evenimpart these characteristics to a compound which is otherwise useless asa flux. The presence of boron trifluoride along with the aforesaidorganic compounds causes the flux to uniformly attack the metal beingsoldered without localized pitting and resultant unsoundness in thesoldered joint. These compounds containing boron trifluoride act at asufiiciently low temperature to permit use with any of the usual softsolders. Furthermore, these flux compounds promote the spread of themolten solder.

The organic compounds which I have found to be useful for making fluxes,as mentioned here-' cyclic ethers, ketones, organic acids, organicesters, amides,- and aliphatic, aromatic, and heterocyclic mono-amines.By the term "unmodified alcohol I mean those monohydric and polyhydricalcohols which contain no other elements than carbon, hydrogen, andoxygen; for example, octyl alcohol, ethylene glycol, glycerin, etc., andare not associated with ethers. The alcohol-ethers are compounds whichcontain both alcohol and ether groups, such, for example, as ethyleneglycol mono-methyl ether and diethylene glycol mono-butyl ether. Aminoalcohols, which are excluded from the foregoing group but which formuseful fluxes when combined with boron trifluoride, are described andclaimed in my copending United States application Serial No. 306,887.Likewise the polyamines are excluded from the above named classes ofmono-amines, the former being described and claimed in my copendingUnited States application Serial No. 306,888. The mono-amines, as theexpression implies, are compounds which contain but one amine group.Compounds of the classes named above which have been successfully usedfor soft solder fiuxing in combination with boron trimono-phenyl ether,maleic, gluconic, oleic, citric,

tartaric, benzoic, and salicyclic acids, acetone, dioxane, digiycololeate, pyridine, quinoline, morpholine, castor oil, mono-, di-, andtri-amyl amines, aniline, ortho-toluidine, and acetamide. Numerousequivalents of the foregoing compounds will suggest themselves to oneskilled in the art.

0f the foregoing compounds, I prefer those belonging to the alcohol, thealcohol-ether and amine classes. When treated with boron trifluoride,the resultant products range in physical form from limpid liquids tosolids, depending on the compound and the amount of boron tri-' fluorideadded, and hence are adapted to a wide variety of applications.Moreover, they may be mixed with conventional vehicles, plasticizers, orother diluents to obtain any desired consistency.

The boron trifluoride combinations with the foregoing organic compoundsmay constitute the entire flux or only a part of it, depending uponthephysical characteristics desired. A useful range within which they maybe employed lies between 1 and 100 per cent of the boron trifluorideaddition compound, where a compound is formed. Although reference hasbeen made to the use of a single compound, it is advantageous attimes toemploy two or more organic substances in combination with borontrlfluoride,

Where two or more of these substances are used, the minimum amount ofeach substance should not be less than about 0.5 per cent of the flux.

The boron trifluoride may combine with the organic compounds inequi-molecular or multimolecular proportions, when the compounds aresaturated with boron trifluoride, that is, one or more molecules of thetrifluoride may be added per molecule of the organic compound, or theremay be one molecule of the trifluoride for two or more molecules of theorganic compound, depending on the character of the substance. In caseswhere compounds are not formed or cannot be readily identified, thetrifluoride is. introduced until a certain increase in weight isobtained. Also, in some cases where compounds are formed, it is mostadvantageous to add a certain amount of the trifluoride to the organiccompound or mixture oif-compounds, and not attempt to saturate them withthe trifluoride so to speak, and thus provide an excess of the organiccompound over and above that united with the boron trifluoride. In suchinstances the proportion of the trifluoride organic compound to theuncombined organic material preferably lies between 50 and '75 per centof the total weight of the flux. Inboth cases, either where a compoundis formed or where one is not formed, the boron trifluorid concentrationmay be conveniently expressed in terms of the weight of the organicsubstance; that is,'the trifluoride represents a certain percentage byweight with respect to the total weight of trifluoride plus the organiccompound. In general, the trifluoride should represent about 0.5 to 50per cent of the weight of Diameter of area in inches covered by M mlsolder Compound Type mired Orig- Oominal pound comwith pound BF:

' I 0.12 Ethylene glycoL. Alcohol Copper iron. M Ethylene glycol Alcoholether do monoisopropyl other. Acetone Ketone Oopper 0.0 Dlglycololeate... Organic esterm. do 0.20 Salicylic acid. Acid Copper iron.Acetamide Amide do 3:3

Mono-amyl Aliphaticamine. .do e. I Morpholine Heterocyclic Copper 0.0

e. Aniline Aromatic amine. do 8.25 Coml. ZnCl, Copperiron. fluxsolution. Coml. ZnCl: ..do paste flux. 23 Rosin ..do ZnClzpaste Oxidized0.47

copper. Rosin do 0.0 Ethylene glycol Alcohol-ethen... .do 0.47

ngnoisopropyl e er.

diluting substance and the attendant mixing operation. Where the fluxconsists of a boron trifluoride addition compound, it should melt'belowand have a boiling point above the fusing temperature of the solderbeing used.

The fluxes herein described can be used with a variety of soft-solders.However, in the soldering oi a particular metal, it is usually desirableto select a solder which will not promote electrolytic corrosion betweenitself and the metal to which it is Joined. a

some examples will serve to illustrate the increase in the spread of thesolder brought about through use of the fluxes described above. In thesetests a comparison was made between the fluxing property of the organiccompound before and after treatment with boron trifluoride. Twocommercial zinc chloride fluxes, a 5 per cent zinc chloride solution anda zinc chloride-Vaseline paste, and rosin, were also included to aflordstandards of comparison by which to judge the spreading powerof the newfluxes. The test consisted of placing a 0.3 gram pellet of 50-50lead-tin solder on a piece of copper or iron sheet about 1 inch squaretogether with about 2 drops of the flux material or the equivalent of 2drops where the material was solid. The fluxes used are listed in thetable below. To show the oxide film penetrating power oi the fluxes,some of the copper samples were oxidized and tested with,

the zinc chloride paste, rosin, and ethylene glycol mono-isopropyl etherboron trifluoride fluxes.

The pieces of sheet with solder and flux placed thereon were then heatedon a hot plate until the solder melted and spread out as far as it wouldgo. Upon removal from the hot plate the approximate diameter of the areacovered by the solder was measured. The data so obtained is given in thetable below. In every instance the organic flux contained no vehicle,plasticizer, or other diluent.

Solder spread tests with BF;-0rgan ic compounds It will be observed thatin every instance the addition of boron trifluoride to the organic comofthe flux, and in some instances, the trifiuoride has actually impartedthis property to the organic compound. Although the zinc chloride fluxcompares favorably with the trifluoride fluxes in causing the solder tospread, it suffers from the important disadvantage of leaving a residuewhich is hygroscopic and therefore, upon being exposed to the air foronly a few minutes,moisture is absorbed and corrosive attack of thesolder and base metal begins immediately. In contrast to such behavior,I have found that the trifiuoride fluxes either leave no residue, or ifany is left, it is non-hygroscopic and substantially non-corrosive. Thetests on the oxidized copper samples show that rosin cannot be usedunder unfavorable metal surface conditions, even though rosin residuesmay be non-corrosive.

I claim:

1. A soft soldering flux composition containing a reaction product ofboron trifluoride and at least one substance from the group consistingof amides, and aliphatic, aromatic and heterocyclic mono-amines, saidflux being characterized by its ability to uniformly attack a coppersurface during a soldering operation and to leave no corrosive residue.

2. A soft soldering flux composition containing from 1 to 100 per centof boron trifiuoride addition compound of at least one substanceselected from the group consisting of amides, and aliphatic, aromaticand heterocyclic mono-amines,

said flux being characterized by its ability to uniformly attack acopper Surface during a soldering operation and to leave no corrosiveresidue.

3. A flux composition consisting of a'boron trifluoride additioncompound of at least one of the substances selected from the groupconsisting of amides, and aliphatic, aromatic and heterocyclicmono-amines, and an excess of said substance with which the borontrifiuoride is not united.

4. A flux composition consisting of to per cent of a boron trifluorideaddition compound of at least one substance selected from the" groupconsisting of amides, and aliphatic, aromatic and heterocyclicmono-amines, and an excess of said Organic substance with which theboron trifluoride is not united.

5. A soft soldering flux composition containing a reaction product ofboron trifluoride and at least one substance selected from the groupconsisting of amides, and aliphatic, aromatic and heterocyclicmono-amines in an amount of 0.5 to 50 per cent of the entire weight ofthe flux, said flux being characterized by its ability to uniformlyattack a copper surface during a soldering operation and to leave nocorrosive residue.

6. A soft soldering flux composition containing a boron trifluorideaddition compound of an aliphatic mono-amine, said flux beingcharacterized by its ability to uniformly attack a copper surface duringa soldering operation and to leave no corrosive residue.

7. A soft soldering flux composition containing a boron trifiuorideaddition compound of an amide, said flux being characterized by itsability to uniformly attack a copper surface during a sglderingoperation and to leave no corrosive resi ue.

8. A method of soft soldering comprising applying a flux. to the metalsurface to be soldered, said flux containing a reaction product of borontrifi-uoride and a substance selected from the group consisting ofamides, and aliphatic, aronnatic and heterocyclic mono-amines, said fluxbeing characterized by its ability to uniformly attack a copper surfaceduring a soldering operation and to leave no corrosive residue, andbringingmolten solder into contact with the fiuxed metal surface.

MIKE A. MIILER.

